Phosphor for light sources, and associated light source

Details Phosphor for light sources, and associated light source Phosphor for light sources, and associated light source

2000-10-23

2003-04-22

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

With luminescent solid or liquid material

Solid-state type

C313S468000, C313S486000, C252S30140R

Reexamination Certificate

active

06552487

ABSTRACT:

TECHNICAL FIELD
The invention is based on a phosphor for light sources and an associated light source in accordance with the preamble of claim 1. It relates in particular to a garnet phosphor which emits in the long-wave range of the visible spectral region and is to be excited by short wavelengths in the visible spectral region. Suitable light sources are in particular a lamp (primarily a fluorescent lamp) or an LED (light-emitting diode), which overall generates white light, for example.
PRIOR ART
WO 98/05078 has already disclosed a phosphor for light sources and an associated light source. In that document, the phosphor used is a garnet of the structure A
3
B
5
O
12
, the host lattice of which, as first component A, comprises at least one of the rare earths Y, Lu, Sc, La, Gd or Sm. Furthermore, one of the elements Al, Ga or In is used for the second component B. The only activator used is Ce.
A very similar phosphor is known from WO 97/50132. The activator used in that document is either Ce or Tb. While Ce emits in the yellow spectral region, the emission from Tb is in the green spectral region. In both cases, the complimentary color principle (blue-emitting light source and yellow-emitting phosphor) is used to achieve a white luminous color with a semiconductor element.
Finally, EP-A 124 175 describes a fluorescent lamp which, in addition to a mercury fill, contains a plurality of phosphors. These are excited by UV radiation (254 nm) or also by short-wave radiation at 460 nm. Three phosphors are selected in such a way that they add up to form white (color mixture).
SUMMARY OF THE INVENTION
The object of the invention is to provide a phosphor in accordance with the preamble of claim 1 which is able to withstand high thermal loads and is eminently suitable for excitation in the short-wave visible spectral region.
This object is achieved through the characterizing features of claim 1. Particularly advantageous configurations are given in the dependent claims.
In detail, according to the invention, a phosphor is proposed for excitation by a radiation source whose emission is in the short-wave optical spectral region. The phosphor has a garnet structure A
3
B
5
O
12
, and it is doped and activated with Ce, the second component B representing at least one of the elements Al and Ga. The first component A contains a rare earth RE selected from the group Y, Sc, Gd, Tb, La and/or Lu, with an amount of at most 5 mol % of A being replaced by praseodymium (Pr). Because of the concentration quenching to be observed with Pr, above all an amount of at most 5 mol % is to be recommended. Particularly favorable is an amount of at most 1 mol %. In this case praseodymium acts as second activator in addition to Ce according to the formula A
3
B
5
O
12
: (Ce, Pr).
Advantageously, the first component A is predominantly (more than 75 mol %) formed by yttrium and/or lutetium, in order to achieve a high efficiency. In addition thereto, it is possible to use amounts of Tb, Sc, Gd and/or La for fine-tuning. Particularly advantageous is the addition of Tb to the component A in small quantities (0.1 to 20 mol %) , since Tb improves the temperature quenching. Good results are furnished, in particular, by a garnet (Y, Tb)
3
Al
5
O
2
: (Ce, Pr).
The phosphor according to the invention can be excited in a wide range of the blue spectral region by radiation in the range from 420 to 490 nm, in particular 430 to 470 nm. A particularly good matching can be achieved to a light source, the peak wavelength of which is in the range 440 to 465 nm.
The phosphor has in particular a garnet structure
(RE
1−x−y
Pr
x
Ce
y
)
3
(Al, Ga)
5
O
12
,
where
RE=Y, Sc, Tb, Gd, La and/or Lu.
The concentration of the two activators should be selected in the following ranges:
0.00005≦x≦0.05;
0.01≦y≦0.2.
The second component B contains advantageously both Ga and Al and may additionally contain In.
It has become evident that the addition of Pr to the YAG:Ce host crystal must be accurately dimensioned, since, when the concentration is too high, the luminous efficacy deteriorates significantly, while, when the dimensioning is too low, a marked effect of the red improvement no longer occurs. A good indication for the calculation of the Pr are its lines which occur in the emission spectrum. In particular the amount of the praseodymium and the condition of the host lattice (selection of the components A and B) should be selected in such a way that, essentially, lines of the Pr below 650 nm appear in the emission spectrum, in particular the two lines of the Pr at 609 and 611 nm. The shorter-wave the red component can be selected, the higher the visual useful effect, since the sensitivity of the eyes decreases very strongly towards long wavelengths. A red component which is obtained by an emission at a wavelength above 650 nm, that is, in a range 650 to 700 nm, is therefore considerably less favorable.
In particular the amount of the praseodymium should be less than 0.3 mol % and should, in particular, be selected to be so small that the two lines of the Pr at 609 and 611 nm appear separated from one another in the emission spectrum. In addition, if the Pr amount is suitably dimensioned, the Pr line at 637 nm can also appear in the emission spectrum. Advantageously the amount of the praseodymium should be greater than 0.2 mol % and should be selected to be so high that the Pr line at 637 nm distinctly appears in the emission spectrum. For, as a result of this, an additional contribution in the red spectral region is obtained, in addition to the two other main lines of the Pr at 609 and 611 nm. It should, in particular, amount to at least 10% of the contribution of the other two lines.
The present invention also comprises a light source which primarily emits radiation in the short-wave blue range of the optical spectral region, this radiation being partially or completely converted into longer-wave radiation by means of a phosphor as specified above. In particular, the primary radiation emitted lies in the wavelength range from 420 to 490 nm, in particular 430 to 470 nm. The primary radiation source used is advantageously a blue-emitting light-emitting diode, in particular based on InGaN, in order to produce a white LED. A particularly good matching of phosphor to primary light source is obtained in the range of a peak emission of the LED in the range 440 to 465 nm. This is achieved by combining a blue LED (primary light source) with a phosphor as specified above, which is excited by the radiation from the LED and the emission (secondary radiation source) from which supplements the remaining blue primary LED radiation to form white light. In the case of using a single Pr-containing phosphor, this phosphor should emit mainly in a broad band in the yellow region of the spectrum and should emit additionally in a narrow band in the red region of the spectrum. In the case of using two Pr-containing phosphors, one of the phosphors should emit mainly in a broad band in the yellow region of the spectrum and should emit additionally in a narrow band in the red region of the spectrum (relatively low Pr content), while the second phosphor has an emission curve which is shifted, relatively to this, towards longer wavelength and has a relatively high Pr content (more than 50% higher than the first phosphor). In particular, the concentration of the Pr in the first phosphor should be selected in such a way that only the two Pr lines at 609 and 611 nm appear, while the concentration of the second phosphor may be selected in such a way that the additional line at 637 nm appears also and furnishes a contribution.
A process for producing the phosphor comprises the following process steps:
Comminution of the oxides and adding a flux;
First annealing in forming gas (mixture of H
2
and N
2
);
Milling and screening;
Second annealing.
According to the invention, for light sources whose emission lies in the short-wave blue spectral region, the phosphor used has a garnet structure A
3
B
5
O
12
and is doped with Ce, the second component

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